Implanted biological prostheses, for example a prosthetic ureter or a prosthetic urinary bladder, require methods and devices to make a connection with the biological structure (that connection is referred to hereinbelow as an anastomosis or as an anastomotic junction). The connection of tissue to a device poses many challenges because living tissue is easily damaged, can be induced to proliferate, or may join to itself and thereby close off the associated passage.
These challenges are demonstrated by canine implantation studies of anastomotic devices, which have exhibited several characteristic failure modes. The simplest of these anastomotic techniques involves the insertion of a tubular part into the biological tube, such as ureter. Between three and ten millimeters of the tip is placed inside the ureteral lumen and secured with permanent sutures. A seal is usually achieved by suturing the resected ureteral edge adjacent to a porous material to which the ureter will become affixed by ingrowth. For such methods, the tip of the inserted tube is tapered, rounded and smooth, since sharp edges or rough surfaces were expected to cause mechanical damage to the tissue. Such mechanical damage should be avoided because it probably initiates inflammation and drives some portions of the wound healing response, for example, the release of growth factors, with subsequent cell proliferation and activity. A similar design to that described above is illustrated in U.S. Pat. No. 4,225,979. Such insert-style anastomoses often fail because of papillary ureteral structures that block the entrance of the inserted tip.
Another prior, unsuccessful mode of anastomosis involves an end-to-end joining, where the perpendicularly resected biological ureter is abutted and directly sutured to a tubular structure of the urinary prosthesis. Canine ureters attached to such devices often develop a closed end, thus preventing transmission of urine to the device.
Attempts with an end-to-end anastomosis of the type used for vascular anastomoses have also failed. With that method matching spatulate end cuts are made on the parts to be attached. The spatulate spreads the sutures over a larger region, thus lessening the chances sutures will rub against each other; it also provides an easy way to join tubes with slightly different diameters. Anastomoses of this type have failed in both of the above-described ways.
In a fourth style of anastomosis, the ureter is inserted into the tubular structure and sutures to the tube are placed in the outer surface of the ureter a few millimeters from the resected end. The distal end of the ureter is allowed to dangle inside the tube. These anastomoses fail as a result of a blind end (cul-de-sac) of biological ureter forming at or just above the prosthetic device.
Because anastomotic failures of the type noted above prevent passage of biological fluids such as urine, the proximal ureter and kidney (in this example) develop hydronephrosis and ultimately the kidney for that ureter will fail.
At least a part of the difficulty of forming an anastomosis seems to be related to natural events, in particular, to peristaltic transport of fluids such as urine and to the epithelial lining, which has an inherent tendency to maintain its continuity. Peristaltic action is visible in the expansion of the diameter of the biologiccal tube such as ureter as a bolus of urine is pushed forward. Peristaltic waves occur regularly with their amplitude and frequency depending upon the amount of fluid such as urine that must be transported. In the case of ureter typically, there are a few waves per minute. As each wave passes, the lumen of the ureter changes from a collapsed, star-like cross-section, where the sides forming the arms of the star touch each other, to an open, polygonal shape, and then returns to the collapsed shape. The muscles in the ureteral wall force the polygonal shape to return to the collapsed shape and thus push the bolus of urine ahead. Because the ureteral wall muscles contract to push the bolus ahead, they force the lumenal surface against any object placed inside the ureter. The peristaltic process can also cause tugging motions on the anastomotic sutures that secure the biological ureter to the device.
Furthermore, the lumen of biological tubes such as ureter is lined with epithelium, a tissue that when cut or damaged, has an inherent propensity to repair itself and proliferate. In particular, when epithelial tissue is disrupted, the natural healing process acts to restore continuity of the epithelial surface. Since the ureter must be cut to connect it to the device that is to carry away urine, this natural process of restoring continuity of the epithelial surface is always initiated as a part of the surgical implantation. Cut epithelial surfaces that are held together for a short time can be expected to heal together and, thus, join.
Other implantable devices offer little guidance in designing anastomoses, such as ureteral ones. Vascular grafts of ureteral size are a current topic of research. In practice, most small vascular grafts are made by transplanting veins or arteries from another site. This method results in primarily a tissue-tissue junction, and hence avoids the problem of connecting tissue to exogenous or man-made materials. Percutaneous devices for the skin, another epithelial tissue, are also plagued by a host of problems, including downgrowth of epithelium which can result in expulsion of the device. One design strategy for percutaneous devices involves selecting materials for implantation and acute care protocols that result in a firm attachment of the collagenous subcutaneous tissue to the device. This strategy is being pursued with the expectation that the firmly attached underlying tissue will limit or frustrate the downgrowth of epithelium.